The proposed studies are part of a continuing investigation of the celluar mechanisms underlying the transport of water, ions, and water soluble non-electrolytes across the epithelial cell layer of the small intestine and other epithelia. Specifically, the investigation is concerned with the kinetics, energetics, and electrophysiology of ionic absorption and secretion by these tissues. Coupling between ionic transfer processes (e.g., sodium and chloride transport) and between these processes and the transport of organic solutes (e.g., sugars, amino-acids and oligopeptides) is also a major concern within the overall scope of the project. Techniques employed include the measurement of transepithelial potential difference, short circuit current, and mucosal membrane potentials in epithelial cells; determination by flame photometry of mucosal sodium and potassium concentrations; measurement of mucosal chloride concentration by electrometric titration, direct determination (with ion-selective microelectrodes) of intracellular sodium, potassium, and chloride activities, and measurement, by radiotracer techniques, of transepithelial and transmucosal fluxes of ions, sugars and amino-acids. Because of their exceptional stability in vitro and their functional resemblance to corresponding mammalian tissues, amphibian (bullfrog and necturus) small intestine and gallbladder are used as experimental preparations.